Low-carbon steels having a yield strength of approximately 170 megapascals (MPa) that exhibit excellent deep drawing behavior are used in a variety of industries, e.g. the automobile industry. However, and despite their forming and cost advantages over high-strength steels, the relatively low-strength level of low-carbon steels results in crash performance of such materials being mainly dependent on a thickness of a sheet thereof. As such, first generation advanced high-strength steels (AHSS) have been developed in order to reduce the weight of automotive components and thereby afford improved vehicle fuel efficiency.
Among first generation AHSS, dual-phase steels are increasingly being used for vehicle components in order to reduce their weight. The excellent strength-ductility balance provide a large formability range and make them one of the most attractive choices for weight saving applications in automobiles.
Dual-phase steels can be produced by subjecting low-carbon steels to an intercritical anneal followed by sufficiently rapid cooling. It is appreciated that an intercritical anneal refers to annealing the steel at a temperature or temperature range below the material's Ac3 temperature and above the Ac1 temperature, i.e. where the microstructure of the steel consists of ferrite and austenite. Also, the rapid cooling of the material transforms the austenite into martensite such that a predominantly dual-phase ferrite-martensite microstructure is produced.
The addition of alloying elements in a low-carbon steel can circumvent the requirement of high cooling rates on a production line in order to obtain martensite as a low transformation product in a ferritic matrix. However, the addition of such alloying elements naturally increases the cost of the steel. In particular, alloying elements such as manganese, chromium, molybdenum, and niobium can be used to reduce the rate of cooling required for the transformation of the austenite to martensite. Also, molybdenum is an effective alloying element that imparts quench hardenability, along with the added benefit of not being prone to selective oxidation during annealing when compared to chromium, manganese, silicon, etc. As such, the use of molybdenum does not hamper the surface characteristics of processed dual-phase steels and affords for improved coating thereof.
Three basic methods are known for the commercial production of dual-phase steels. First, an as-hot-rolled method produces a dual-phase microstructure during conventional hot rolling through the control of chemistry and processing conditions. Second, a continuous annealing approach typically takes coiled hot- or cold-rolled steel strip, uncoils and anneals the steel strip in an intercritical temperature range in order to produce a ferrite plus austenite microstructure/matrix. Thereafter, rapid cooling higher than a critical cooling rate for the steel chemistry is applied to the strip to produce the ferrite-martensite microstructure. Finally, a batch annealing approach simply anneals coils of hot- or cold-rolled material.